Natural leathers have the unique texture and feel with adequate moisture absorbency as well as unique gradual moisture and air permeability. They are made of entangled colagen fibers that are unique to them and have higher toughness as compared with other cloth materials.
Natural leathers are, however, limited in supply and some are quite scarce while others are extremely expensive. Natural leathers in general are usually more expensive than other woven or knitted fabrics made of natural or synthetic fibers. Natural leathers are further defective in that as they are supplied in the shape and size of their original animals, they not only require cutting and sewing operations but are inconvenient in providing larger-sized products with consistent texture. Still further, when left standing under moist conditions for an extended period of time, they tend to become swollen and too soft or become stretched, losing the shape and strength.
Various types of artificial and synthetic leathers have therefore been developed and used to substitute natural leathers. Such artificial and synthetic leathers are grained or embossed to give as natural appearance as possible such as that of crocodiles or snakes. Aggregates with moisture absorbency are often admixed to give them excellent feel and moisture permeability without tackiness similar to that of natural leathers. Micropores are also formed in them to obtain similar effect.
As a typical example, it is known to apply water miscible organic solvent solution of polyurethane elastomer on a substrate made of knitted, woven or non-woven fabric, immerse the substrate in water to remove the solvent as well as to coagulate polyurethane to form a porous layer on the substrate. Artificial or synthetic leathers with porous layers thus formed on the substrate are, on one hand, comparable to natural leathers in draping property, texture and feel and have excellent moisture permeability. On the other hand, they lack strengths such as wear resistance, and thus cannot be used as they are. The porous layer formed on the substrate by any one of the above mentioned methods is irregular in its pore size, shape and distribution depending on the conditions. The above mentioned methods are therefore not satisfactory for manufacturing artificial and synthetic leathers with uniform moisture permeability and water resistance on a mass production basis. Moreover, because of inferior wear resistance, conventional artificial and synthetic leathers must be reinforced on the surface by applying a surface treatment agent comprising an organic solvent solution of polyurethane resin on the porous layer formed on the substrate and heating and drying the same. This, however, entails a defect that the moisture permeability of the porous layer will be hampered by the reinforcement layer.
An attempt is therefore made to manufacture artificial or synthetic leathers having both the wear resistance and moisture permeability by gravure coating said reinforcement layer in dots on the porous layer. However, the resultant product is insufficient both in the wear resistance and moisture permeability; and the moisture permeability is not uniform over the entire surface of the product.
It has also been proposed to manufacture artificial or synthetic leathers by mixing powder of natural skin/leather in a resin solution such as of polyurethane resin and applying the solution on the surface of a substrate. Alternatively, said powder of natural skin/leather is mixed in a resin material such as polyurethane resin to manufacture leather-like sheets and films. Still another method comprises the use of a solvent solution mixed with the powder of natural skin/leather to give leather-like finish on the paint layer, coating layer and the like. There is an attempt to wash out a portion of natural skin/leather powder contained in the coating layer and the like.
Notwithstanding excellent moisture absorbency and feel comparable to that of natural leather, conventional artificial and synthetic leathers are defectively low in moisture permeability, if any at all. The moisture absorbency itself is limited to the extent which the natural skin/leather powder contained in the product can absorb, and it is not possible for the product to continuously absorb a large amount of water. Another defect of the conventional products of artificial or synthetic leathers is that as it is not possible to completely wash out the skin/leather powder contained therein other than the soluble components, the products will not attain porous structure.
With the foregoing in the background, the development of artificial and synthetic leathers in the form of sheets, films and coating layers having sufficient moisture permeability, water resistance and excellent wear resistance is long awaited. Requirements of such products to be developed are that the pores giving rise to moisture permeability be finer and uniform in size and that these pores be uniformly and more densely distributed. The products to be developed are required to have moisture absorbency as well, if necessary.
None of the requirements are met in the artificial and synthetic leathers according to the conventional methods in which the natural skin/leather powder is mixed in the course of production, or in which a porous layer is formed by applying a water-miscible organic solvent solution of polyurethane elastomer on the substrate and by immersing in water to wash out the solvent as well as to solidify polyurethane.
The present inventors conceived an idea of admixing hydrophilic and water-soluble gelatin into a solution of synthetic resin paint or a solvent solution of synthetic resin to be used in manufacturing synthetic resin paint layer, coating layer, film or finished cloth. This method of admixing hydrophilic and water-soluble gelatin into synthetic resin paint solution or synthetic resin solvent solution is advantageous in a number of aspects.
For one, gelatin to be admixed in the paint solution or solvent solution of synthetic resin is incorporated in such products as paint layer, coating layer, film or finished cloth instead of becoming dissolved in the solvent. Hydrophilic and water-soluble gelatin incorporated in the products exhibits excellent moisture absorbency and is easily dissolved by hot water, whereby the resultant products will have moisture permeability imparted by gelatin incorporated therein. It is also possible to wash out hydrophilic and water-soluble gelatin incorporated in the products with hot water to leave pores in the products.
The present inventors therefore made an attempt to form paint layers and films by admixing hydrophilic and water-soluble gelatin into paint solution or solvent solution of synthetic resin. Our experiments demonstrated that gelatin to be admixed in the synthetic resin solution must be an ultrafine powder of at least smaller than 30 microns in particle size to obtain excellent paint layers and the like products.
When the particle size of gelatin to be admixed is larger than 30 microns, the resultant products such as films and resin layers formed on the finished cloths can not be formed thin. As a result, the products will not have soft feel or sufficient moisture permeability because of the thickness.
When the thickness of the films or the resin layer formed on finished cloths is reduced, gelatin particles admixed in the resin would come out to project from the surface, giving rugged feel. Cracks would also develop from the periphery of the projecting gelatin particles, and the film starts to tear.
When gelatin particles are eluted from thin films and the like, large pores will be left, making the products brittle and deteriorating the water resistance. Water is also found to infiltrate into the products from the surface.
It was therefore concluded from our experiments that the particle size of gelatin powder to be admixed in the paint solution or solvent solution of synthetic resin for forming paint layers, films and the like must be in the range smaller at least than 30 microns.
An attempt was therefore made to pulverize hydrophilic and water-soluble gelatin into particles of less than 30 microns in size (the shorter diameter is cited herein in the case of elongated particles). Hydrophilic gelatin was thoroughly dried and subjected to experimental pulverization in a small amount using a pulverizer with sufficiently dry atmosphere, to obtain ultrafine powder of substantially 30 microns in size. However, when the water content of the gelatin to be pulverized increased, gelatin particles became sol and fused with one another, making it impossible to continue pulverization. Particularly when pulverized in a pulverizer such as a ball mill, gelatin particles were observed to become fused with one another with time. It was also found that this fusion of gelatin particles takes place as the particles absorb the water contained in the pulverizer to become sol. This is more markedly observed when the temperature inside the pulverizer rises.
In forming such products as paint layers and finished cloths using ultrafine gelatin powder of approximately 30 microns, gelatin particles were found to remain in the liquid in the form of flocculates instead of being dispersed when the particle size is smaller than 30 microns. The reason for gelatin particles in flocculates to remain in the solution is not known. It is assumed from the fact that the solution has a high viscosity that flocculates of gelatin particles are enveloped by the viscous liquid and thus prevented from dispersing.
Unless gelatin is subjected to pulverization using a special experimental apparatus, gelatin particles under pulverization in air will absorb the water contained in the air to become sol especially because the temperature inside the pulverizer rises and become fused with one another, making micronization impossible.
Another attempt was made to dry and cool gelatin and pulverize thus dried and cooled gelatin using a special apparatus. It is noted that use of this special apparatus is more costly and is not therefore suitable for pulverizing gelatin in large volume.
Under such circumstances, the present inventors have developed a method for pulverizing gelatin in dimethylformamide or an organic solvent admixed with dimethylformamide, said method being characterized in that gelatin particles do not become sol by absorbing moisture during pulverization and that pulverized gelatin is easily eluted with hot water.